1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to forming fabrics for the forming section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The properties of absorbency, strength, softness, and aesthetic appearance are important for many products when used for their intended purpose, particularly when the fibrous cellulosic products are facial or toilet tissue, paper towels, sanitary napkins or diapers.
These products can be produced using a variety of processes. Conventional manufacturing machines include a delivery of the suspension of cellulosic fiber onto one or between two forming fabrics. This partially dewatered sheet is then transferred to a press fabric, which dewaters the sheet further as it transfers the sheet to the surface of a large Yankee dryer. The fully dried sheet is either creped or not as it is removed from the Yankee surface and wound onto rolls for further processing.
An alternative process employs a through air drying (TAD) unit either replacing the press fabric above with another woven fabric which transfers the sheet from the forming fabric to the through air drying fabric. It is this fabric which transfers the sheet to a TAD cylinder where hot air is blown through the wet cellulosic sheet, simultaneously drying the sheet and enhancing sheet bulk and softness.
Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam.
The present invention relates specifically to the forming fabrics used in the forming section. Forming fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to form and convey the paper product being manufactured to the press section.
However, forming fabrics also need to address water removal and sheet formation issues. That is, forming fabrics are designed to allow water to pass through (i.e. control the rate of drainage) while at the same time prevent fiber and other solids from passing through with the water. If drainage occurs too rapidly or too slowly, the sheet quality and machine efficiency suffers. To control drainage, the space within the forming fabric for the water to drain, commonly referred to as void volume, must be properly designed.
Contemporary forming fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric woven from monofilament and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The design of forming fabrics additionally involves a compromise between the desired fiber support and fabric stability. A fine mesh fabric may provide the desired paper surface and fiber support properties, but such design may lack the desired stability resulting in a short fabric life. By contrast, coarse mesh fabrics provide stability and long life at the expense of fiber support and the potential for marking. To minimize the design tradeoff and optimize both support and stability, multi-layer fabrics were developed. For example, in double and triple layer fabrics, the forming side is designed for support while the wear side is designed for stability.
Those skilled in the art will appreciate that fabrics are created by weaving, and having a weave pattern which repeats in both the warp or machine direction (MD) and the weft or cross-machine direction (CD). It will also be appreciated that the resulting fabric must be uniform in appearance; that is there are no abrupt changes in the weave pattern to result in undesirable characteristics in the formed paper sheet. Due to the repeating nature of the weave patterns, a common fabric deficiency is a characteristic diagonal pattern in the fabric. In addition, any pattern marking imparted to the formed tissue will impact the characteristics of the paper.
To generate bulk, cross directional tensile, absorbency, and softness in a sheet of paper, a fabric will often be constructed so that the top surface exhibits plane differences between strands. For example, a plane difference is typically measured as the difference in height between two adjacent weft (cross direction) strands in the plane of the forming surface. Bulk, cross directional tensile, absorbency, and softness are particularly important characteristics when producing sheets of tissue, napkin, and towel paper. Hence, tissue forming fabrics preferably exhibit plane differences in the forming side.
One attempt to provide a plane difference is shown in U.S. Pat. No. 5,456,293. The '293 patent shows a single layer TAD fabric wherein the MD yarns are interwoven to produce a zigzag effect. However, as stated in the '293 patent's abstract, the pattern has an array of pockets extending diagonally in alternating fashion across its width. Although the '293 pattern does distribute these pockets, it is preferable to minimize the effects of any discernible pocket patterning.
Additionally, several other patents disclose single layer fabrics having plane differences; e.g. U.S. Pat. No. 5,806,569, U.S. Pat. No. 5,839,478, and U.S. Pat. No. 5,853,547. While all of these patents describe fabrics exhibiting a plane difference in the forming side, their single layer designs do not allow for the optimized balance between support and stability that multi-layer fabrics can provide.
Therefore, a need exists for a tissue forming fabric having a plane difference in the forming side to generate bulk, cross directional tensile, absorbency, and softness in the tissue paper while minimizing the adverse effects of a strongly defined diagonal pocket pattern.
A further need exists for such a fabric to provide more cross-directional stability and stiffness to prevent cross directional fabric shrinkage, improve sheet formation and appearance, and potentially increase life.
The present invention is a multi-layer tissue forming fabric having different diameter, size, or shape weft strands to produce a plane difference on the forming side. The present invention provides a solution to the problems of providing a fabric pattern having a plane difference while maintaining good sheet fiber support and fabric stability properties.